Rigid polyurethane and urethane-modified polyisocyanurate foams are in general prepared by reacting a stoichiometric excess of polyisocyanate with isocyanate-reactive compounds in the presence of blowing agents, surfactants and catalysts. One use of such foams is as a thermal insulation medium in, for example, appliances.
In appliance formulations, generally, polyether polyols are used as isocyanate-reactive compounds. Polyester polyols impart excellent flame retardancy characteristics to the resulting polyurethane foams, and, hence, are generally used for building applications.
One of the most important characteristics of rigid polyurethane foam is the thermal insulation properties, i.e. the lambda value, which has to be as low as possible, especially at 10° C., when the foam is used as insulation in appliances. This low lambda value should also be reached with the nowadays used physical blowing agents, especially the hydrocarbon ones (e.g. cyclopentane).
Therefore, it is an object of the present invention to provide a process for making rigid polyurethane foams based on polyester polyols showing a low lambda value, especially at 10° C., and also that can make use of notably cyclopentane.